Time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA) are techniques which allow a communications system to accommodate a large number of system users. CDMA has recently been used in the United States for digital cellular telephone systems, and is being adopted for this purpose in other regions such as Japan and Europe through a variation of CDMA known as wideband CDMA (WCDMA). CDMA uses a spread spectrum modulation technique, in which the signal energy of each channel is spread over a wide frequency band, and in which multiple channels each corresponding to a different system user occupy the same frequency band. CDMA offers the advantage of efficient use of the available frequency spectrum, but at the cost of being computationally intensive.
In order to demodulate a received signal, a mobile CDMA receiver must identify and synchronize to a local base station in a timely manner. This process is known as acquisition. During acquisition, the mobile receiver determines the spreading code sequence and spreading code phase of a suitable base station. To make it easier for the mobile receiver to acquire the spreading code sequence and phase of the base station, the base station transmits several pilot signals. The pilot signals are helpers which allow the mobile receiver to more easily determine the spreading code sequence and spreading code phase. In order to synchronize to the base station, the mobile station selects a possible synchronization point and tests whether the signal energy using this synchronization point exceeds a threshold. This process is called hypothesis testing. The mobile receiver must perform hypothesis testing using different possible synchronization points until it finds one with a very high probability of being correct. The mobile receiver also continually searches for other base stations as call handoff candidates.
Thus acquisition in a mobile CDMA receiver requires many computations. These computations tend to decrease battery life. A searcher receiver which performs faster searches and therefore consumes less power would be desirable. Such a searcher receiver is provided by the present invention, whose features and advantages will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.